As cellular communications become more widely used, the number of individual users and calls multiplies. Increase in cellular communications utilization magnifies the opportunity for interference between the different users on the cellular system. Such interference is inevitable because of the large number of users and the finite number of cellular communications cells (cells) and frequency bands (channels) available.
As originally implemented, cellular communications systems were broken down into omni-trunks where each cell was able to use each channel in a 360.degree. radius. Because of overlap in the area serviced by cells, a caller utilizing one cell in the penumbra between two cells could interfere with a caller utilizing the other cell if both were on the same channel. To avoid this interference the channel utilized by a caller in one cell would have to be disabled for any other callers in any adjacent cells. Disabling such a channel on all adjacent cells would cause many more cells than actually utilized to have the caller's channel unavailable for use by other callers. Such disabling of channels to avoid interference was recognized to lead to under-utilization of cell resources as well as depletion of available channels.
In order to avoid such under-utilization, reuse patterns were adopted in the art wherein different channel sets are assigned to different cells so that callers in adjacent cells tend not to utilize the same channel concurrently. Problems with such reuse patterns, however, include difficulty in creating a cell reuse pattern utilizing channels in such a way as not to have any two cells' use of a channel overlap, as well as limitations on the number of channels available for use in implementing such a reuse pattern.
To reduce the interference problems caused by other users in the omni cell 360.degree. configuration, cells have also been broken down into 120.degree. sectors such that each channel available at the cell only communicates in an area of 120.degree. radial coverage about the cell. An advantage, in addition to the reduction of interference realized by the sector system, is that such a cell achieves extended range as compared to an omni cell 360.degree. system simply due to the ability to focus a greater signal gain on the antennas. Individual cells may then cover a larger area, and communications signals may be stronger within the cell.
A problem with going from the omni cell 360.degree. configuration to the sector system, however, is that as a result of splitting of the cell into 120.degree. sectors only a third of the channels are available in each sector. This results in a reduced call capacity, i.e., reduced trunking efficiency, in any particular cell sector at a cell as compared to that available in the omni cell 360.degree. configuration. This is because if all of the channels in a particular sector are currently being utilized by subscriber units, a channel available in another sector in that same cell may not be available for utilization by a new caller located in the loaded sector. For example, if an omni cell has 60 channels and a sector system is divided into three 120.degree. sectors, each sector only has 20 channels. If in sector 1 there are 20 channels being used and a twenty-first user attempts to gain access, this user will not have access to the cell because of a lack of available channels in the sector even if sectors 2 or 3 have available channels. Whereas, in the omni cell 360.degree. configuration, provided that all 60 channels are not being utilized, the twenty-first user would have had access to the cell because all channels are by definition potentially available throughout the cell.
Trunking efficiency is a measure of the number of users which can be offered a particular grade of service with a particular configuration of fixed channels. As demonstrated above, the way in which channels are grouped can substantially alter the number of users handled by a trunked system.
Of course one solution to the increased blocked calls experienced due to decreased trunking efficiency might be to add to the total number of channels at the cell. However, this solution is undesirable in that the addition of channels further complicates establishing cell re-use patterns. Furthermore, as the number of channels per sector increases the possibility of interference events also increases. Likewise, the addition of channels increases the energy density within the cell and thus reduces the carrier to interference ratio which results in poorer signal quality.
It shall be appreciated that loading of sectors is often cyclic or dynamic in nature rather than constant. For example, during certain times of day, such as business commuting times, a particular sector, such as a sector encompassing an urban highway, may service more users than during other times of day. Therefore, during particular times a particular sector or sectors may require increased capacity in order to service all users whereas at other times the cell's capacity might be better utilized when spread more homogeneously throughout the cell's coverage area.
It would, therefore, be advantageous to make more efficient use of cellular capacity by being able to make sectors dynamically shapable in order to provide increased capacity to a particular area within the cell's radiation pattern by making more channels potentially available to that particular area, without actually increasing the total number of channels within the cell. Ideally, the shapable sectors will be composed of narrow beams so as to provide a convenient means by which sectors may be sized radially about the cell. Systems implementing such narrow beams are described in U.S. Pat. No. 5,563,610, entitled "NARROW BEAM ANTENNA SYSTEMS WITH ANGULAR DIVERSITY," incorporated herein by reference, and the associated continuation-in-part U.S. Pat. No. 5,643,968 entitled "NARROW BEAM ANTENNA SYSTEMS WITH ANGULAR DIVERSITY," also incorporated herein by reference. Management of such a system, including concurrent beam and channel management within a neighborhood of cells, is disclosed in U.S. Pat. No. 5,745,841 entitled "METHOD AND APPARATUS FOR IMPROVED CONTROL OVER CELLULAR SYSTEMS," incorporated herein by reference.
Another problem in the art is that in a cellular system, communications are typically mobile, often in vehicles traveling at considerable speed. Such mobile communication devices tend to travel through the various sectors and/or cells of a cellular system, thereby continuously effecting signal quality as fringe or shadow areas are entered and exited. These effects of signal quality are not limited to the mobile communication device itself, but also effect other communication devices operating in the area. For example, a communication device operating in one cell, although experiencing acceptable signal quality itself, may in fact be causing interference for another communication device. Such interference may be in the form of co-channel interference, near/far problems, increased energy density and the like. Therefore, it is desirable to provide a means by which such a communication device may be handed off to another sector or cell, although its communication parameters do not necessitate the hand off, in order to better serve another communication device. Likewise, such a communication device may be experiencing communication of a quality so as to be within acceptable parameters although communication of a better quality may be had through an adjacent sector or cell.
Recognizing the mobility of communications and the attendant communication quality issues, therefore, it would also be advantageous to be able to dynamically shape sectors in their longitudinal, or lengthwise, reach from a cell site. Preferably, as it is determined that a communication device is causing interference for another communication device or as it is determined that this communication device may itself be better served by another sector or cell, the shape of the sector currently serving the communication device may be adjusted to force a hand off of the communication device to another sector or cell. Ideally, the longitudinal shape of sectors will be accomplished through the use of attenuators in the receive signal path.
A need therefore exits in the art for a system and method for providing cell sectors adapted to provide for greater trunking efficiency and the ability to serve more users, such as through dynamically adjusting the shape of the sectors and/or providing for their overlap. Moreover, a need in the art exists for such a system to provide azimuthal as well as longitudinal shaping of the sectors.